1. Field of the Invention
The present invention relates to an electrical circuit system that uses a D.C. power source such as a battery.
2. Description of Related Art
In various devices that use batteries as a power source, control of the various circuit blocks of the devices is generally conducted by a CPU such as a microcomputer. In this case, the same battery supplies power to the CPU as well as to the various circuit blocks.
FIG. 7 is a block diagram showing an example of a device that uses a battery as the power source. With this device, a DC-DC converter 2, a strobe charging circuit 3A and other circuits 4 are connected to the battery 1, and power is supplied from the battery 1. Furthermore, the CPU 4A is connected to the output of the power supply circuit 14. Charging by the strobe charging circuit 3A is controlled by a signal input into the CTL terminal. In other words, the strobe charging circuit 3A is controlled by the output of an AND gate 8, which finds the logical sum of the output of a comparator 6, which monitors the voltage of the battery 1 using a standard voltage source 5 as a standard and uses the output of a latch circuit 7 that is controlled by the CPU 4A. In addition, on the DC-DC converter 2 side of comparator 6 is a back-up capacitor 9 which adds a capacitive component to the output charge of the DC-DC converter.
With this kind of device, when the strobe charging circuit 3A moves to a charging state, the voltage from the battery 1 starts to drop because of the high current load of the strobe charging circuit 3A. Furthermore, when the battery voltage drops as far as the minimum voltage needed to maintain the output of the DC-DC converter 2, it becomes difficult to guarantee proper operation of the CPU 4A which is connected to the output of the DC-DC converter. Consequently, a standard voltage (supplied by a standard voltage source 5) for the comparator 6, which detects the battery voltage, is set at a voltage higher than this minimum voltage. When the battery voltage drops below the standard voltage, the CTL terminal of the strobe charging circuit 3A is controlled via the AND gate to halt the charging operation of the strobe charging circuit 3A. When the charging operation is halted, the battery voltage rebounds, and when the voltage has rebounded above a certain hysteresis amount, the charging operation is started again.
FIGS. 9(a)-9(b) are waveform diagrams showing the state of charging or not charging of the strobe charging circuit 3A. Here, FIG. 9(a) shows the waveform of the strobe charging circuit 3A, while FIG. 9(b) shows the waveform of the battery voltage.
The battery voltage waveform fluctuates dramatically when this kind of charging control is conducted on the strobe charging circuit, as shown in FIG. 9(b). Consequently, when the strobe charging circuit frequently repeats the operations of charging and not charging, the charging time is lengthened, and because the charging time increases, the user may become impatient. The other circuits 4, which use the same power source as the strobe charging circuit, also experience large voltage fluctuations. The battery voltage drops a large amount when the charging current is largest, immediately following strobe illumination. When circuit 4 includes display circuits such as LEDs that supply light to displays in the viewfinder, the LED brightness dims. If instead, in order to avoid this problem, the displays inside the viewfinder are extinguished during charging, the display information is not available at all in the viewfinder.